TPM not needed for BitLocker?

I have some Win 8 laptops which all have BitLocker running.

On some laptops with TPMs, I chose the pin# method.
However, one laptop without TPM, still supports BitLocker except I use long passwords instead.
But no "USB Key" needed like the one for Win 7 laptop without TPM.

Does Win 8 inherently support BitLocker with/without TPM?

Does TPM somehow make only using PIN# more secure?

Thanks.